Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (MEA) interposed between gas diffusion backings. The MEA and gas diffusion backing arrangement is, in turn, interposed between anode and cathode flow field/current collector plates. The MEA includes a solid polymer electrolyte membrane (PEM)--also referred to as an ion exchange membrane or proton exchange membrane--having electrodes on either side thereof. The electrodes may be attached directly to the PEM membrane. Alternatively, the electrodes may be attached to or integrated into the gas diffusion backings, and the backings pressed against the PEM membrane. Gas diffusion backings are typically fabricated of porous, electrically conductive sheet materials, such as carbon/graphite fiber paper or carbon/graphite cloth. These gas diffusion layers often incorporate micro diffusion layers. A catalyst layer, typically in the form of platinum or platinum supported on carbon particles, is located at each membrane/electrode interface to induce the desired electrochemical reactions. The electrodes are typically coupled to one another to provide an electrical path for conducting electrons between the electrodes to an external load. Hydrophobic enhancement materials, such as Teflon.RTM., may be incorporated into the fuel cell to aid with product removal.
At the anode, a fuel-reducing reagent reacts at the catalyst layer to form cations. These are thermodynamically driven through the membrane toward the cathode. At the cathode, oxidant reagent reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form reaction product. In electrochemical fuel cells employing hydrogen (or hydrogen-containing gas) as the fuel, and oxygen (or oxygen-containing gas) as the oxidant, the reaction at the anode produces hydrogen cations, or protons, from the fuel supply. The proton exchange membrane facilitates the migration of hydrogen protons from the anode to the cathode. In addition to conducting hydrogen protons, the membrane acts as a gas separator, generally isolating the hydrogen-containing fuel stream from the oxygen-containing oxidant stream (although nominal gas cross-over does occur). At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane, forming water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are as follows:
Anode reaction: EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.-
Cathode reaction: EQU 1/2+L O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O
Typically, the MEA and gas diffusion backings are interposed between a pair of electrically conductive fluid flow field plates, or collector plates, each having at least one flow passage formed therein. These flow field plates are typically fabricated from graphite or metal. The flow passages, or channels, direct the fuel and oxidant to the respective electrodes; namely, fuel to the anode and oxidant to the cathode. The plates act as current collectors, provide structural support for the electrodes, provide access channels for transporting the fuel and oxidant to the anode and cathode, respectively, and provide channels for the removal of product water formed during operation of the cell.
Hydrogen transport through the PEM requires the presence of water molecules within the membrane. Consequently, maintaining adequate membrane hydration is critical. In addition to maintaining adequate ionic conductivity and proton transport, uniform membrane hydration prevents localized drying, or hot spots, resulting from higher localized resistance. Overall, dehydration may impede performance, increase resistive power losses and degrade the structure of the membrane. In conventional fuel cells, membrane hydration is achieved by humidifying the fuel and oxidant gases prior to their introduction into the fuel cell.
One commonly-used method for pre-humidifying fuel cell gas streams is to employ membrane-based humidifiers. Where membrane-based humidifiers are employed, reactant humidification is achieved by flowing the respective gases on one side of a water vapor exchange membrane while flowing deionized water on the opposite side of the membrane. In such arrangements, water is transported across the membrane to humidify the fuel and oxidant gases. Another known technique for pre-humidifying the reactant gas streams comprises exposing the gases directly to water in an evaporation chamber to permit the gas to absorb evaporated water. Yet another known pre-humidification technique comprises directly injecting or aspirating water into the respective gas streams before introducing them into the fuel cell.
Generally, pre-humidification is undesirable because it requires auxiliary fuel cell components, increasing the relative complexity of fuel cell systems. For instance, pre-humidification generally requires dedicated components for storing and transporting water. Additional components may also present system reliability issues. For example, where fuel cells are operated in sub-freezing conditions, water solidification can result in the weakening of mechanical components. Auxiliary water storage and transport components also reduce operating efficiency and add to the overall cost of the system.
For the foregoing reasons, the need exists for a PEM fuel cell assembly capable of maintaining hydration of the fuel cell membrane without requiring additional components for humidifying reactant streams prior to their introduction into the fuel cell stack.